I'm going to make an attempt to try to summarize the tax implications on Hayward's/Celtics decision tomorrow or so, but I can already see by the numbers that they can be quite severe.
With quite severe I mean that if Hayward opts in (around $35M, I like rounded numbers) and we extend him in 2021, we're basically going to have to pay like an additional $4.5M to the tax man for every $1M extra we give Hayward in salary (due to repeater tax).
For instance if in summer/autumn 2021, Hayward signs for $20M a year instead of $15M, the extra costs for the following two seasons could be: $10M (2*5M) in salary + $45M (estimated 20M and 25M) in taxes = $55M in total.
My estimate is that the Celtics owners would save about $70M (at least) in total if they could get Hayward to agree to opt out and take a guaranteed 80/4 extension, instead of opting in and signing an extension in 2021 at an average of 60/3. Thus $80M (guaranteed money, covered for injury risk) over $95M ($35M guaranteed).
Any extension we give to Hayward (assuming he'll be paid in the 15-25M range) will cost between 100-180M in total for 2022 and 2023 together (so 50-90M a year) under the assumption that the 'salary' core (Walker, Tatum, Brown, Smart) stays intact.
If Hayward opts out (can't see him having the open market to make that a legit option) or if we trade him after he opts in for a lower expiring contract and duck under the cap (manageable) then the Celtics save like an additional 25M immediately and also postpone potential repeater taxes with 2 years.